There is a number that predicts how long you’ll live independently, and no wearable on the market tracks it. It’s not your VO2 max. It’s not your resting heart rate or your sleep score. It’s how hard you can squeeze. Grip strength has quietly become one of the most respected biomarkers in longevity research. Large population studies going back to the PURE study — nearly 140,000 adults across 17 countries — found that grip strength predicted death from cardiovascular disease better than systolic blood pressure. Researchers now describe it as a “single vital sign of aging,” a proxy for total-body…